Ewald Cress continues his dive into system internals, this time looking at SOS_Mutex:

Put differently, we can build a mutex from an auto-reset EventInternal by tacking on an owner attribute, making a rule that only the owner has the right to signal the event, and adding assignment of ownership as a fringe benefit of a successful wait. A nonsignalled event means an acquired mutex, and a signalled event means that the next acquisition attempt will succeed without waiting, since nobody currently owns the mutex. The end result is that our SOS_Mutex class exposes the underlying event’sSignal() method and its own take on Wait(). From the viewpoint of the mutex consumer, the result of a successful wait is that it owns the mutex, and it should act honourably by calling Signal() as soon as it is done using the resource that the mutex stands guard over.

There’s some deep detail here, so this is definitely one of those “after your first cup of coffee” posts to read.

Related Posts

Understanding Hash Match Aggregates

Itzik Ben-Gan continues his series on grouping and aggregating data by looking at the hash match aggregation process: The estimated CPU cost for the Hash Aggregate in the plan for Query 8 is 0.166344, and in Query 9 is 0.16903. It could be an interesting exercise to try and figure out exactly in what way […]

Read More

Row Width And Snapshot Isolation

Kendra Little shows us the impact that row width has on snapshot isolation: So I went to work to demonstrate row width impact on the version store — when only a tiny bit column is changed in the row. Here’s how I did the test: I created two tables, dbo.Narrow and dbo.Wide. They each each have a […]

Read More


June 2016
« May Jul »